Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

["Semwal, Sumit" <sumit.semwal@ti.com>]

Hi Daniel, Rob,


On Tue, Dec 6, 2011 at 3:41 AM, Rob Clark <rob@ti.com> wrote:
> On Mon, Dec 5, 2011 at 3:23 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Mon, Dec 05, 2011 at 02:46:47PM -0600, Rob Clark wrote:
>>> On Mon, Dec 5, 2011 at 11:18 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>>> > In the patch 2, you have a section about migration that mentions that
>>> > it is possible to export a buffer that can be migrated after it
>>> > is already mapped into one user driver. How does that work when
>>> > the physical addresses are mapped into a consumer device already?
>>>
>>> I think you can do physical migration if you are attached, but
>>> probably not if you are mapped.
>>
>> Yeah, that's very much how I see this, and also why map/unmap (at least
>> for simple users like v4l) should only bracket actual usage. GPU memory
>> managers need to be able to move around buffers while no one is using
>> them.
>>
>> [snip]
>>
>>> >> +     /* allow allocator to take care of cache ops */
>>> >> +     void (*sync_sg_for_cpu) (struct dma_buf *, struct device *);
>>> >> +     void (*sync_sg_for_device)(struct dma_buf *, struct device *);
>>> >
>>> > I don't see how this works with multiple consumers: For the streaming
>>> > DMA mapping, there must be exactly one owner, either the device or
>>> > the CPU. Obviously, this rule needs to be extended when you get to
>>> > multiple devices and multiple device drivers, plus possibly user
>>> > mappings. Simply assigning the buffer to "the device" from one
>>> > driver does not block other drivers from touching the buffer, and
>>> > assigning it to "the cpu" does not stop other hardware that the
>>> > code calling sync_sg_for_cpu is not aware of.
>>> >
>>> > The only way to solve this that I can think of right now is to
>>> > mandate that the mappings are all coherent (i.e. noncachable
>>> > on noncoherent architectures like ARM). If you do that, you no
>>> > longer need the sync_sg_for_* calls.
>>>
>>> My original thinking was that you either need DMABUF_CPU_{PREP,FINI}
>>> ioctls and corresponding dmabuf ops, which userspace is required to
>>> call before / after CPU access.  Or just remove mmap() and do the
>>> mmap() via allocating device and use that device's equivalent
>>> DRM_XYZ_GEM_CPU_{PREP,FINI} or DRM_XYZ_GEM_SET_DOMAIN ioctls.  That
>>> would give you a way to (a) synchronize with gpu/asynchronous
>>> pipeline, (b) synchronize w/ multiple hw devices vs cpu accessing
>>> buffer (ie. wait all devices have dma_buf_unmap_attachment'd).  And
>>> that gives you a convenient place to do cache operations on
>>> noncoherent architecture.
>>>
>>> I sort of preferred having the DMABUF shim because that lets you pass
>>> a buffer around userspace without the receiving code knowing about a
>>> device specific API.  But the problem I eventually came around to: if
>>> your GL stack (or some other userspace component) is batching up
>>> commands before submission to kernel, the buffers you need to wait for
>>> completion might not even be submitted yet.  So from kernel
>>> perspective they are "ready" for cpu access.  Even though in fact they
>>> are not in a consistent state from rendering perspective.  I don't
>>> really know a sane way to deal with that.  Maybe the approach instead
>>> should be a userspace level API (in libkms/libdrm?) to provide
>>> abstraction for userspace access to buffers rather than dealing with
>>> this at the kernel level.
>>
>> Well, there's a reason GL has an explicit flush and extensions for sync
>> objects. It's to support such scenarios where the driver batches up gpu
>> commands before actually submitting them.
>
> Hmm.. what about other non-GL APIs..  maybe vaapi/vdpau or similar?
> (Or something that I haven't thought of.)
>
>> Also, recent gpus have all (or
>> shortly will grow) multiple execution pipelines, so it's also important
>> that you sync up with the right command stream. Syncing up with all of
>> them is generally frowned upon for obvious reasons ;-)
>
> Well, I guess I am happy enough with something that is at least
> functional.  Usespace access would (I think) mainly be weird edge case
> type stuff.  But...
>
<snip>
>
>> On the topic of a coherency model for dmabuf, I think we need to look at
>> dma_buf_attachment_map/unmap (and also the mmap variants cpu_start and
>> cpu_finish or whatever they might get called) as barriers:
>>
>> So after a dma_buf_map, all previsously completed dma operations (i.e.
>> unmap already called) and any cpu writes (i.e. cpu_finish called) will be
>> coherent. Similar rule holds for cpu access through the userspace mmap,
>> only writes completed before the cpu_start will show up.
>>
>> Similar, writes done by the device are only guaranteed to show up after
>> the _unmap. Dito for cpu writes and cpu_finish.
>>
>> In short we always need two function calls to denote the start/end of the
>> "critical section".
>
> Yup, this was exactly my assumption.  But I guess it is better to spell it out.

Thanks for the excellent discussion - it indeed is very good learning
for the relatively-inexperienced me :)

So, for the purpose of dma-buf framework, could I summarize the
following and rework accordingly?:
1. remove mmap() dma_buf_op [and mmap fop], and introduce cpu_start(),
cpu_finish() ops to bracket cpu accesses to the buffer. Also add
DMABUF_CPU_START / DMABUF_CPU_FINI IOCTLs?
2. remove sg_sync* ops for now (and we'll see if we need to add them
later if needed)
>
> BR,
> -R
>
>> Any concurrent operations are allowed to yield garbage, meaning any
>> combination of the old or either of the newly written contents (i.e.
>> non-overlapping writes might not actually all end up in the buffer,
>> but instead some old contents). Maybe we even need to loosen that to
>> the real "undefined behaviour", but atm I can't think of an example.
I guess that should be acceptable for our video / media use cases. How
about other potential users of dma-buf? [I am asking this because
Jesse did tell me that there were some other subsystems also
interested in dmabuf usage]
>>
>> -Daniel
BR,
~Sumit.
>> --
>> Daniel Vetter
>> Mail: daniel@ffwll.ch
>> Mobile: +41 (0)79 365 57 48
<snip>

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